I have a friend who diy'ed a button on single webpage that he presses on his phone while at work in order to open the gate to the building that he lives so delivery people can get in. I also think Bret Victor diy'ed the AC as mentioned while he was a student quarter century or more ago[0]
I know this sounds like the famous "just do it this way in linux instead" criticism of Dropbox back in the day. But I do think we reached "life parodies fiction" with these smart devices where it makes sense to give diy another go. And with AI, there's less excuses this time around I would imagine.
But I would literally rather buy a cheap phone, a cheap SIM, hotspot it and connect it to a charger and have the AC connect to that and isolate it that way instead of letting it touch the network.